Motor vehicle outer panels and the like are overcoated often by the so-called two-coat one-bake method wherein a base coat composition and a clear coat composition are applied to the substrate wet-on-wet and then cured at the same time by heating.
As clear coat compositions for use in the two-coat one-bake method, thermosetting coating compositions are generally used which consist primarily of an acrylic resin or like hydroxyl-containing resin and a melamine resin.
However, air pollution due to sulfur oxides, nitrogen oxides, etc. has become aggravated on a global scale in recent years to produce an acid rain, which has developed a new drawback in the overcoat formed on motor vehicle outer panels by the two-coat one-bake method using the clear coat composition, i.e., susceptibility to etching, whitening or staining. It is urgently required to obviate this drawback. The overcoat has another drawback in that the surface thereof is subject to scratches, for example, when the motor vehicle is washed.
On the other hand, a two-coat one-bake coating process is also proposed with use of a clear coat composition which comprises an epoxy- and hydroxyl-containing resin and a cyclic anhydride serving as a crosslinking agent (U.S. Pat. Nos. 4,732,790 and 4,732,791). This coating process gives coatings that have inferior scratch resistance and do not always have satisfactory acid resistance.
The present invention provides a novel two-coat one-bake coating process for forming overcoats that are free of the foregoing deficiencies of the prior art and form overcoats that have excellent acid and scratch resistance.